1. Field of the Invention
The present invention generally relates to image processing devices, and particularly relates to an image processing devices which compresses and encodes color images.
2. Description of the Related Art
In the field of data processing and data communication, data compression and encoding techniques are widely used for the purpose of reducing memory size required for storing image data and of shortening transmission time when transmitting image data.
In image forming devices such as digital copiers, there is a need to perform editing processes at high speed within limited memory space, such editing processing including rotation, combination, superimposition, size change, etc., of images, and fixed-length compressing and encoding techniques are employed to this end. This is because a fixed-length encoding results in memory addresses of stored encoded data being easily identified with respect to a position of interest within the image to be edited. Further, in the case of digital copiers having a function of electric sorting, there is a need to store a plurality of images in a memory, resulting in a need for large memory size.
In general, the amount of information representing a monochrome image is smaller than the amount of information representing a color image. Use of appropriate encoding schemes results in a higher coding rate being obtained for the monochrome image. Devices for processing color image data do not always process color images, but occasionally process monochrome images. Applying the same encoding technique to monochrome images as applied to color images is not desirable in terms of coding efficiency. Accordingly, it is desirable to use different encoding schemes for monochrome images and color images, respectively.
An image coding device disclosed in Japanese Patent No. 2,520,891 checks whether each block obtained by dividing an input image is a monochrome image or a color image. If a block is ascertained as a monochrome image, brightness information of the coded image data can be stored in an increased amount as color information does not exist in a monochrome image. Further, the memory space spared by absence of color information may be used for storing brightness information of the next block. If not necessary to exploit all the spared space, filler bits may be placed without a concern for a decrease in efficiency.
Further, an image coding device disclosed in Japanese Patent No. 2,618,944 increases the number of quantization bits for brightness information in monochrome blocks so as to have the same number of quantization bits for one monochrome block as for one color block. Alternatively, the number of quantization bits for brightness information in monochrome blocks is set to half the number of quantization bits for brightness information in color blocks, thereby adjusting the total number of bits in image blocks.
The image coding devices disclosed in Japanese Patent No. 2,520,891 and in Japanese Patent No. 2,618,944 employ different encoding schemes between monochrome images and color images, so that separate encoding means are necessary for respective images. Further, an improvement in coding efficiency is attained only when an image is comprised of a predetermined single color, so that the field of application is not as wide as desired. When an encoding technique that outputs a shorter length of codes for monochrome (black and white) images is applied to a blue only image, such an encoding technique does not produce a shorter length of codes, failing to achieve an improvement in coding efficiency. If an attempt is made to check the color of the image, an extra time becomes necessary for such data processing, thereby hampering efforts toward high-speed encoding processing. If the distribution of color components is measured throughout the entire image prior to commencement of the encoding processing, for example, the start of encoding processing must be delayed by the time length required for such data processing.
Further, when a single image contains monochrome blocks as well as color blocks, the resulting codes for each block may differ from block to block. Namely, features of fixed-length coding are lost, with increased difficulties in performing various editing processes on the image data.
Accordingly, there is a need for an image processing device which can encode image data including color images and monochrome images at high speed and at high coding rate.